The development and parameter optimization of CVD and etch processes used in the manufacture of microelectronic devices has heretofore been accomplished to a large extent by empirical techniques, such as the use of monitor and send-ahead wafers. The use of such techniques has been necessitated because no reliable method has existed to measure and control certain key process parameters, such as the deposition rate and quality of a deposited film, as they are changing during the process, i.e., in real-time. This problem has been particularly true in those CVD processes which use precursor gases derived from liquid or solid sources to form the deposited film. For example, CVD processes which use tetraethoxysilane (TEOS) to deposit oxide films have attained widespread use as described in U.S. Pat. No. 4,849,259 to Biro et al. Condensible gases have also frequently been used as a source of phosphorous or boron in the formation of doped oxide films. Such condensible feedgases are frequently derived by bubbling a carrier gas through a reservoir of liquid precursor or passing a carrier across a solid precursor held at a temperature necessary to maintain an adequate vapor pressure. The gas entrained in this manner is then delivered to a reaction chamber through heated delivery lines (see the aforementioned U.S. Pat. No. 4,849,259). In an alternate method, undiluted vapor may be delivered to a reaction chamber without the use of a carrier gas. Applicants have discovered that the quality of the film deposited and the film deposition rate in such CVD processes are a function of the concentration of the feedgas. The feedgas concentration is in turn a sensitive function of the thermal stability of the liquid source and the delivery apparatus, the level of the liquid in the reservoir and the rate of flow of the carrier gas. Thus in the absence of a means to measure the concentration of such condensible feedgases in real-time, it has heretofor been necessary to employ costly and time consuming empirical techniques to control such CVD processes.